1. Field of the Invention
The present invention relates to casting procedures, particularly those used to manufacture molded articles of metal substance and preferably precision parts made from metal alloys. In particular, the present invention is directed to a method which involves the regulation of a casting cycle using low-pressure techniques.
2. Discussion of Background Information
Various processes for manufacturing molded parts, particularly cast alloy parts, under low-pressure conditions are known.
Low-pressure casting is a known foundry technique in which the bottom of a metallic or non-metallic mold is filled with a metal or a liquid alloy, placed in a hermetically sealed furnace, and solidified. The metal can rise within the mold by means of an injection tube. The filling is performed with the assistance of discharge fluid introduced into the furnace under a pressure of several decibars. After filling the mold, an excess deadhead pressure is maintained during solidification of the material. Non-solidified material is recovered from the bottom of the mold in the injection canals as soon as solidification of the part has occurred and after the discharge pressure has been stopped.
In this technique, any of the following molds can be used:
metallic molds,
molds made of sand, or of various materials (graphite, zirconium, carborundum) whose grains are bonded by a binder (generally, this binder is a synthetic resin), or
molds made out of ceramic or plaster.
The metallic molds are strong but expensive and are only used, as a result, for large series.
Non-metallic molds have a comparatively reduced cost. They furthermore have the advantage of adjustable permeability, and permit satisfactory filling of the depression.
This low-pressure casting technique using molds of inexpensive sand is particularly adapted to the new needs of the industry, particularly in the aeronautic field, which necessitate the production of medium series of molded alloy parts of high mechanical quality, which have delicate and defined tolerances.
The technical problems of casting affecting the quality of the products concerned are principally:
control of metal turbulence during its elevation in the mold, which turbulence relates to the speed of evolution of the metal and which determines its oxidation;
protection against ram knocks which can occur during the establishment of deadhead excess part pressures (that is to say, of compensation for their retraction) which can lead to encrustation of the metal between the grains of the mold;
non-premature occurrence of solidification;
evolution of the metal (in structure, in displacement, in cooling, etc.) conforming to the thermal need of the casting;
reproduction of operations making it possible to make the quality of parts produced uniform; and
improved efficiency of the performance of the tasks.
So as to better understand the principle of operation of the process according to the invention, it should be noted that when the "casting front" of the metal is positioned in a quasi-static fashion at a level H, above the level of the metal in the crucible, the delivery pressure into the crucible is P=H.rho.g (.rho. being the volumetric mass of the metal considered and g representing the coefficient of acceleration of gravity). As soon as there is movement of the liquid, breakage forces occur between the metal and the walls.
Experience and calculations show that a differential law of variation of the discharge pressure is thus obtained by the formula: EQU (dP/dt)=K.rho.g(dH/dt)=K.rho.gV
(V is the vertical speed of elevation of the casting front and K is .gtoreq.1 and is a coefficient taking into account frictions which depend on the geometry of the mold and on V).
For low values of speed V, and thus of dP/dt, K=1.
For large values of V, K and thus V tend to an asymptotic value.
In all which follows, we place ourselves in the most common case where V is low, one thus has during the filling phase: EQU P=.rho.gH and dP/dt=.rho.gV,
while in the excess pressure phase: EQU P=.rho.gH+.alpha.P,
.alpha.P being the overpressure phase undergone by the metal at the upper portion of the mold.
When the metal is in the excess pressure phase, this excess pressure depends upon the level of metal in the crucible, by means of the term H, this latter varying with the succession of castings.
Taking into account the conditions of theoretical law concerning casting, as set forth above, necessitates:
on one hand, acting on the discharge pressure of the metal, during the dynamic phases, such that the casting front progresses regularly and follows precise speed characteristics. Whatever the shape or the sharpness of the depressions to be filled, this progression must occur without suddenly slowing down, which causes too rapid solidification of the liquid mass, and sudden interruption of solidification before it is completed, and also without turbulences adapted to cause oxidations which result in weaknesses or localized discontinuities in the parts being cast,
on the other hand, quickly applying to the metal, after it has filled the depression, excess pressures which are substantial enough to compensate for retraction in the course of solidification, but in varying conditions such that they do not cause penetration of metal between the grains of the mold, and
finally, carrying out these actions by taking into account random disturbances, such as lowering of the metal level in the crucible and gas leaks.
The prior art attempted in vain to universally solve these problems as follows. In certain systems described to this date, the casting cycle follows phases limited by reference points situated in the depression. Flowing is caused by admission to the crucible of constant streams of air determined in advance. The speed of flow of the metal is thus only a generally unpredictable consequence of these flows, of the geometry of the parts and of the unavoidable gas leaks. Other systems impose a speed of constant variation of pressure over the entire cycle, or further carry out an adjustment at several levels of pressure so as to obtain a predetermined final pressure. These systems do not correct the pressure to take into account drops in the level of the metal. This prevents reproduction of the castings. Finally, certain systems perform a correction based upon given indications at the beginning of the sequence, particularly with an analogue computer. But his requires a preliminary adjustment and excludes the possibility of casting different parts in each cycle as has often been the case in the aeronautical field. Furthermore, the corrections performed suffer from imprecision in their evaluation and the errors committed, in general, only grow with successive castings.